Switch mode power supplies are well known. In an isolated switch mode power supply using an input transformer with isolated primary and secondary windings, step-down regulation is needed to supply the load with a desired rated voltage. Step-down regulation in the secondary stage of such an SMPS in commonly obtained by using an inductor to increase the output impedance of the source. For example, FIG. 1 shows the secondary stage of a known SMPS. An alternating current (AC) input is applied to primary winding 20 of transformer 21. The secondary winding 22 is connected to rectifiers 23 and 24 which are connected to direct current (DC) capacitors 25 and 26. The output of capacitors 25 and 26 is connected as shown to an output circuit 17 comprising high side and low side switches 27 and 28, respectively, which may be MOSFETs operated to alternately open and close with a timing determined to maintain a predetermined fixed voltage on output capacitor 29. Such circuits require the inductor 30 to increase the output impedance of the source, thereby operating as a charge storage element to smooth the switched output of the transistor switches.
The inductor 30 is a large, inefficient component. It would be desirable to provide an SMPS circuit which does not require an inductor in the output circuit.